Step 3: Side trim
Since my math never works out, I decided it's best to just measure every piece as you go, then you know it will be right. So turn the bedframe upside down, and place the 1x10 pine pieces in their respective spots (make sure you label the foot of the bed so you don't get it confused later. Even with our precise, superb measuring, you could still be off by a half inch or something and create some headaches.
Do it the old-fashioned way
But the board up to the end of the 4x4. Draw a line in pencil on the other end of the 4x4. There's your cutting line. Do this for the other side. Then do this for the foot of the bed, factoring in the two outside pieces of pine, for a nice flush fit (or make it a miter if you're more talented than me). I actually somehow screwed this up and the piece at the foot of the bed was about half an inch off (measure once, cut twice...), so the side pieces ended up being longer, extending to the edge of the foot. This is noticable and not that cute, but it still works. (You could say it "adds character" or some other bull).
Time for some good ole glue action
Using the foot of the bed as the first piece, get it ready so that it won't be too difficult to set the clamps and line up the board exactly to the top of the legs (Also, if you haven't figured it out, flip the frame back right side up). Then, apply wood glue to the edge along the legs and the frame where the pine will meet. Set it, clamp it tightly in place, and then firing up the nailgun, throw some nails in there to help hold it in place. Wipe the excess glue off, and let it dry for 20 min or so and you can remove the clamps and move on to the side pieces. The nails will do their job the rest of the way.
To do the side pieces, I wanted to prop it up on an edge, just to make it a little easier to reach, and have fewer glue runs. It's a good idea. Now, finish the two side pieces and take a break for a beer.